Certain industrial processes utilize, in their workings, solutions containing ions of metals. A broad spectrum of metals are included in various solutions for use in a wide variety of processes. At some point in many industrial processes utilizing metal ion-containing solutions, it has been necessary to remove metal ions from the process streams. From time to time, these streams are so-called spent process, purge or waste streams discarded while generally including a quantity of the metal ions. Where the spent process streams have been discarded, these metal ions have been thereby unrecoverably lost. Yet, for some process streams containing somewhat elevated metal ion quantities, continued use of the stream within the process may be imperiled by the presence of the metal ions. In the past, these metal ions discarded from the process or contained in unrecycleable spent process streams have represented, depending upon the metal, a potential health or environmental problem. Where the metal ions have had significant value, such as gold, silver, copper, and nickel, unrecoverable metal ions in these process streams also have represented a considerable economic loss.
Often other chemicals in these process streams would have continued usefulness in the industrial process and could be recycled but for the presence of the metal ions. Where otherwise useful process chemicals are discarded due to metal ion content, the economic and environmental cost of discarding these chemicals compound losses associated with simple discarding of the metal ions.
Over the years, economic and social costs associated with unrecoverable metal losses in the handling of process streams has stimulated a variety of proposals for their recovery. Where concentration of electrowinnable metal ions in a process stream is in excess of about 10 grams per liter, it has been found that the process stream can be electrolyzed efficiently to recover the metal. A substantial portion of metals contained in such process streams has been thereby recovered. Considerable difficulty has been encountered in reducing efficiently the metal ion concentration in a typical process stream below about one gram metal per liter utilizing only simple electrolytic cell treatment, particularly where the fluid stream is relatively nonelectrically conductive. Therefore, depending upon the volume and initial metal ion concentration of the process stream, electrowinnable metal remaining in many process stream applications has remained considerable even after electrowinning treatment.
Where it is desired that residual metal ion concentrations in a process stream be reduced substantially below one gram per liter, a large number of procedures have been proposed wherein the metal ions are caused to react with a chemical to produce a salt of the metal ion which precipitates from the metal ion-containing process stream. Significant problems have been encountered with this approach including: costs associated with the precipitation chemicals; difficulty in recovering the metal from the precipitated salts; the potentially dangerous nature of reactants used to precipitate the metal ions; toxicity and other problems with the precipitate; and in some cases, difficulties disposing of potentially objectionable new ions introduced into the process stream by the chemical reaction.
Removal of metal ions from fluid streams utilizing an ion exchange resin is known. Problems with ion exchange recovery have included difficulty in recovering a considerable portion of the metal ions from a fluid used to regenerate the resin and difficulties in efficiently rendering the regeneration fluid suitable, i.e., sufficiently low in concentration of the metal ion to enable reuse in further resin regeneration.
More recently a particular type of electrolytic cell has been developed, one in which small particles circulate within en electrolyte stream flowing in a zone between an anode and a cathode current feeder. The particles occasionally contact the cathode current feeder and function as the cathode in the electrolytic cell. These so-called particulate bed electrolytic cells (PBC) have been found capable of efficiently removing metal ions from a fluid stream containing metal ions in an initial concentration of about one gram per liter to a concentration as low as about 50 parts per million (ppm). When attempting to achieve a final concentration of 50 ppm or less, these PBCs have been found to operate somewhat inefficiently on streams having an initial metal ion concentration significantly below one gram per liter. PBC operation in a process stream to a very low residual metal ion concentration, for example, less than 50 ppm, contributes to reduced electrical current efficiency of the PBC in overall recovery of the metal ions. Therefore PBCs have not provided a wholly satisfactory route to achieving a final metal ion concentration approaching one ppm in a process fluid, particularly in situations where the initial metal ion concentration in the process fluid is significantly below one gram per liter or where electrical conductivity of the fluid stream is low.
Heretofore, reasonably efficient electrolytic recovery to a residual metal ion concentration significantly below 50 ppm in a purge stream of significant volume has not generally been practical using electrolytic techniques.